The document discusses bistable digital logic devices for all optical circuits. It begins with background on how optical architectures could replace electrical ones to overcome limitations of Moore's Law. It then introduces an optical logic gate scheme using a photonic resonant tunneling device based on a photonic crystal nanocavity. This device exhibits single wavelength bistability due to saturable absorption, which could be used to implement digital logic gates. The document also discusses two wavelength bistable switching and concludes that while optical devices are not yet as fast as electronic ones, emerging photonic crystal technologies may enable faster and more efficient all optical computing.

1. Bistable Digital Logic Devices Halim
Bistable Digital Logic Devices
for All Optical Circuits
Mohammad Faisal Halim (Faissal)
Laser Course
Professor Dosinville
1

2. Bistable Digital Logic Devices Halim
Bistable Digital Logic Devices
for All Optical Circuits
Contents
Page
Topic Number
Prologue: Moore’s Law 3
Background: The Optical Architecture 5
Introduction: The Optical Logic Gate 6
Utilizing a Photonic Resonant-Tunneling Device
Based on Photonic-Crystal Nanocavity 8
Single Wavelength Bistability 8
Conclusion 13
Appendix 14
2

3. Bistable Digital Logic Devices Halim
Bistable Digital Logic Devices
for All Optical Circuits
Prologue: Moore’s Law
Figure 1: Moore’s Law
The relentless push for Moore’s Law has resulted in microchip circuit components that
are becoming so small that any further push towards miniaturization (in order to attain
more speed) will result in quantum mechanical effects becoming more pronounced in the
3

4. Bistable Digital Logic Devices Halim
devices. These effects will not just result in power losses, but will also result in the
devices having less reliable behavior: their behavior will acquire a statistical nature.
Further, with the current state of electrical technology copper (which is used in the
interconnects, in chips) is approaching its information carrying capacity.
The Harry Truman Approach: “If you can’t stand the heat, get out of the kitchen.” That
quote is one way to justify moving out of the current paradigm to chip design, entirely,
and migrate to an inherently faster one: using light to carry information. The dream of
optical computing has existed for a long time, but the technology has been a long time in
coming. The basic concept is this: use light, instead of electricity, to transmit information
across a chip, and use logic gates that switch light, instead of currents and voltages, to
perform logic operations. The primary advantage is that light travels much faster than a
current, so chips that use light will be a lot faster, and optical switching could be done
faster than current and voltage switching (which is done in modern transistor
technology), thus speeding up processor speeds.
4

5. Bistable Digital Logic Devices Halim
Background: The Optical Architecture
A computer microprocessor utilizing all optical circuitry (which would not be “circuitry”
as taught in introductory physics) would have dielectric waveguides as to channel the
light inside the processor, and optical transistors and other optical bistable devices to do
the logic operations.
Figure 2: Source: MODELING OF PHOTONIC BAND GAP STRUCTURES AND PROPOSED SYNTHESIS SCHEMES
By Srivatsan Balasubramanian, RPI, 2002
Figure 3: Source: Hwang, Cho, Kang, Lee, Park, Rho, “Two-Dimensional Optical Interconnection Based on Two-Layered Optical
Printed Circuit Board,” IEEE Photon. Technol. Lett., vol. 19, no. 6, pp 411-413, Mar. 15, 2007.
5

6. Bistable Digital Logic Devices Halim
Introduction: The Optical Logic Gate
The following schematic shows a digital logic circuit, where the inputs and outputs are all
light, rather than voltages or currents.
Figure 4: Schematic of a digital logic device
The way all optical technology is being developed, all inputs and outputs to such a device
will have to be lasers. This is because while technologies like MEMS can be used to
switch light, these systems will be cumbersome, and will not have the desired speeds that
motivated all optical computing, in the first place. Laser is used, instead, for theoretical
and experimental work in this area for the following reasons:
1. Laser light being monochromatic, can be controlled using structures that have a
photonic band gap (PBG).
2. Laser light can be confined into tight spaces, thus allowing miniaturization of
devices (if small enough wavelengths are used).
3. With technologies like opal and inverse opal based photonic crystals (PCs) that
have a PGB at the wavelength of the laser, the laser light can be confined in
microcavities that have a high Q, thus allowing for high intensities of lasers being
generated within devices.
4. It is practical to generate high enough intensities of laser light in microdevices to
utilize non-linear optical (NLO) effects, for use in optical switching.
6

7. Bistable Digital Logic Devices Halim
5. With PCs the spontaneous emission of light of a chosen frequency can be
prevented in a microcavity, thus allowing for high efficiency, low threshold
lasers, for use in all optical devices.
Currently, a number of hybrid technologies are being developed that use electro-optic
effects, but they are limited (in speed) in the long run, since they are still dependent on
the speed of electronics. All optical devices, on the other hand, will only be limited by the
speed of light in the dielectrics used, and the speeds with which the materials used in
such devices will react to the light.
7

8. Bistable Digital Logic Devices Halim
Utilizing a Photonic Resonant-Tunneling Device
Based on Photonic-Crystal Nanocavity
According to the paper “Optical bistable switching action of Si high-Q photonic-crystal
nanocavities,” (Notomi, Shinya, Mitsugi, Kira, Kuramochi, Tanabe, OPTICS EXPRESS
Vol. 13, No. 7 Apr. 4, 2005) single wavelength and two wavelength bistable devices an
be made for al optical circuits, to implement digital logic. The device the group used is
shown in Figure 5.
Figure 5: Photonic resonant-tunneling device based on PC nanocavity
Single Wavelength Bistability
Figure 6 shows that the light transmission characteristics of the material under test varies
with the intensity of the incident light: as the intensity of the light (at any given
wavelength) increases, at a certain intensity the transmitted intensity goes up, and when
the incident light is being lowered in intensity then at a certain intensity the transmitted
intensity goes down.
8

9. Bistable Digital Logic Devices Halim
Figure 6: Utilizing intensity dependent transmission curves
Figure 7 shows a schematic of what happens in Figure 6, for any chosen wavelength.
Figure 7: A schematic of Figure 6, for a chosen wavelength.
9

10. Bistable Digital Logic Devices Halim
What Figure 7 shows is this (the numbers match the numbers on the curves):
1) As the incident intensity is swept down then at some intensity the transmitted
power drops down precipitously.
2) The point marked ‘2’ in the figure corresponds to the intensity at which the device
is in the OFF state.
3) Then, as the incident intensity is increased then at a certain intensity that is greater
than the intensity at which the device turned OFF the device turns ON, that is the
transmitted intensity increases dramatically.
The effects shown in Figure 7 are due to an effect called “Saturable Absorption” (Laser
Fundamentals, Silfvast). Figure 8, perhaps, gives a better depiction.
Figure 8: Output signal versus input signal, showing bistability
10

11. Bistable Digital Logic Devices Halim
For a device, as depicted in Figure 4, an input (called “Signal” in Figure 4) could come in
at an input intensity slightly less than I1(in) (from Figure 8), and the control signal
(“Control,” from Figure 4) could have an intensity such that the total intensity of the two
beams is slightly greater than I2(in) (from Figure 8), and the resulting output signal
(“Output”, from Figure 4) could be the output from an AND gate, as used in digital logic.
In principle, this sort of bistability is achieved by a saturable absorber. As the incident
intensity is increased, the beam is absorbed by the absorber, until at a certain intensity
[I1(in) (from Figure 8)] the absorber is bleached, ad so it suddenly lets the light through,
with very little loss in intensity. When the incident laser is being reduced in intensity then
the absorber will still have some stored photons, causing it to stay bleached until the
incident beam is reduced in intensity to I2(in) (from Figure 8) – this is lower than I1,
when the absorber looses its stored photons and the transmitted intensity goes down.
Since I2(in) is less than I1(in) (both, from Figure 8) we see the hysteresis.
11

12. Bistable Digital Logic Devices Halim
The group also performed two wavelength bistable switching, and obtained the following
result (which gets conceptually more complicated), as shown in Figure 9 (for the device
shown in Figure 5).
Figure 9: Digital Logic
Figure 10 shows the two wavelength switching times and energies.
Figure 10: Switching speeds and energies
12

13. Bistable Digital Logic Devices Halim
Conclusion
Although all optical bistable switching devices are still not as fast as all electronic
devices, they have the potential to be made faster, and more energy efficient, particularly
in light of emerging technologies in fabrication of PCs that have 3D bandgaps.
Some PBGs are shown in Figure 11.
Figure 11: Photonic Bandgaps and material construction of the PCs
13

14. Bistable Digital Logic Devices Halim
Appendix
“Optical bistable switching action of Si high-Q photonic-crystal nanocavities,”
(Notomi, Shinya, Mitsugi, Kira, Kuramochi, Tanabe, OPTICS EXPRESS Vol. 13, No.
7 Apr. 4, 2005)
MODELING OF PHOTONIC BAND GAP STRUCTURES AND PROPOSED
SYNTHESIS SCHEMES, By Srivatsan Balasubramanian, RPI, 2002
Hwang, Cho, Kang, Lee, Park, Rho, “Two-Dimensional Optical Interconnection Based
on Two-Layered Optical Printed Circuit Board,” IEEE Photon. Technol. Lett., vol. 19,
no. 6, pp 411-413, Mar. 15, 2007.
Intel Corporation
Laser Fundamentals, Silfvast
14